b) What makes you think a 20mm cannon can stop the God tier zombie? That thing is between 5 and 6 times as tall as an average person and even thicker by ratio. Just by simple division that means your gauge is equivalent to 3.5-4mm in relation to an average human sized zombie, effectively you're trying to kill this thing with the gun attached in the picture.

Furthermore, 5-6 times the size of a human is over 3 times the size of an average elephant, which is what a 20mm gun is generally designed to have stopping power against. Couple that with the fact that it takes even more comparative stopping power to take out a zombie, and your base is looking pretty roflstomped.

c) No spare parts

d) You clearly have no idea how a septic tank works

e) I have a couple friends in HVAC, it's 4 in the morning here so I won't bug them now, buy I'll get back to you on whether or not your HVAC system is up to snuff

6/10, blows the competition out of the water in terms of size and thought, but considering that you have clearly considered this possibility before, I expect you to have put more thought into the details.